Film resistor having nonconductive coat and method of making the same



Dec. 30, 1969 F. M. COLLINS 3,486,988

FILM RESISTOR HAVING NONCONDUCTIVE COAT AND METHOD OF MAKING THE SAME Filed June 7, 1965 F IG. 2

- 9 F IG. 3

POWER SUPPLY 4,

INVENTOR FRANKLYN M. COLLINS ATTORNEY United States Patent Office 3,486,988 Patented Dec. 30, 1969 US. Cl. 20438 2 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a tin oxide film resistor having a nonconductive tin oxide protective coating and to the method of making the resistor and of controlling its ultimate resistance value while simultaneously anodizing the nonconductive tin oxide coating over the conductive film.

This invention relates to resistors and method of making resistors for use in electrical circuits, and in particular to metal oxide film resistors which are made by depositing conductive metal oxides on nonconductive substrates.

Resistors have been made by applying thin metallic films to nonconducting base materials such as by sputtering or evaporative techniques. Conducting metals with relatively low ohmic resistances such as gold, platinum, palladium, nickel, etc., have been used in the manufacture of such resistors. These metals are deposited in exceedingly thin films. For higher resistance values the film thickness is only silghtly greater than the molecular sizes of the metal with the resistance characteristics resulting from the small cross-sectional area of the metal. These thin films deposited on relatively thick base materials are subject to deterioration from electrical currents and from temperature variations, as well as from mechanical stresses and abrasion. A method of producing these metal film resistors which have particular values of resistance is to deposit a relatively thick metal film on the substrate and thereafter anodize the film to reduce its thickness, which in turn reduces the cross-sectional area of the film until the particular resistance value desired is obtained. This method, which is disclosed in United States Patent No. 3,148,129, in addition to enabling one to obtain very thin film having high resistance, also forms a protective film over the surface of the resistors which precludes substantial variations in resistances which might otherwise occur due to contamination of the resistor surface.

More recently developed resistive films are metal oxide films which have been found to be conductive and which may be deposited on nonconductive substrates. Examples of such metal oxide film resistors are given in United States Patent No. 3,044,901. As shown in this patent, a protective coating such as lacquer or other suitable material must be applied to the metal oxide coating. It has been found that many protective coating adversely affect the metal oxide film due to ion conduction and resistivity changes in the protective film. One method of overcoming this adverse effect caused by the protective coating is to deposit a nonconductive metal oxide coating over the conducting metal oxide coating to insulate the conducting film from the protective coating. Such an arrangement is shown in United States Patent No. 3,134,689. The prior art method of depositing the nonconductive insulating coating is to use a deposition method similar to the method in which the conductive coating was deposited on the base, i.e. by brushing, spraying, stenciling, or silk screening, which substantially increases the cost of the resistor.

In addition to the foregoing difficulty, the resistance of the conductive coating of prior art metal oxide film resistors could only be adjusted by precision control of the method by which the metal oxide is applied to the substrate or by subsequent removal of some of the conductive metal oxide such as by mechanical spiraling. As shown in United States Patent No. 3,012,383, when such mechanical means are used to remove conductive material, a smooth coat is virtually impossible and the jagged edges left by the spiraling operation produce resistors of varying resistance values and unstable characteristics.

I have eliminated the above-mentioned difficulties by discovering that metal oxide films may be electrolytically anodized, as in the case of prior art metal films, to produce a surface layer on the conducting metal oxide film which is relatively nonconducting and will act as a protective coating for the conductive portion underneath. By monitoring the resistance value while anodizing, it is possible to precisely adjust the resistance to any value.

It is an object of the present invention to provide improved film resistors and a method of making the same. It is another object of the present invention to provide film resistors which are extremely reliable. It is still another object of the invention to provide a method of adjusting the value of metal oxide film resistors to precision values. It is a further object of the invention to provide a metal oxide film resistor which has a protective essentially nonconductive coating and a method of making such coating by anodizing the metal oxide films. Other objects, features, and advantages of this invention will be readily apparent from consideration of the following specification relating to the annexed drawings in which:

FIG. 1 is a sectional view of a substrate having a conductive metal oxide coating applied thereto to form a resistor;

FIG. 2 is a cross-sectional view of the resistor of FIG. 1 after the metal oxide film has been anodized to the desired value; and

FIG. 3 is a schematic diagram of an apparatus suitable for electrolytic anodizing of the resistor of this invention.

Referring to the drawings, FIG. 1 is a sectional view of a resistor having a nonconductive substrate 2 and a metal oxide coating 3 deposited on the substrate. It is understood that the shape of the substrate is immaterial to my invention and may be flat or cylindrical or any other shape which is commonly used in the resistor art. Terminal means 5 are suitably attached to opposite ends of the coating 3 to facilitate connection to electrical circuits.

FIG. 2 shows the resistor element of FIG. 1 after it has been subjected to anodization. The thickness of the anodized layer 4 is determined by thickness of the crosssection of the conducting oxide 3 which is necessary to produce the desired resistance value. If low resistance values are needed, then a relatively thin anodized coating would normally be formed. The coating must, however, be of sufiicient thickness to provide the insulating protective coating desired. It is understood that the thickness of the original conducting metal oxide film will determine the amount of anodization necessary to obtain specified resistance values.

FIG. 3 is a schematic representation of a suitable means for electrolytically anodizing the tin oxide coating. A tank 6 contains an electrolytic bath 7. The electrolyte 7 may consist of sulfuric acid, either undiluted or diluted by up to water by volume. Another suitable electrolyte is potassium hydroxide dissolved in a ratio of 1.1 g. potassium hydroxide per ml. of water. A cathode 12 is immersed in the electrolyte and is connected by lead 11 to one side of power supply 9. The tin oxide resistor serves as the anode in the solution and is shown at 8 connected by lead to power supply 9.

The following is a detailed description of the anodization of a particular metal oxide film resistor:

A tin oxide film of about 5000 A. in thickness was suitably deposited on an inert solid cylindrical insulating substrate to form a resistor. The conductive film may be deposited by the methods disclosed in United States Patent No. 3,134,689. The initial resistance of the resistor as deposited was 38.91 ohms. A 500 ml. beaker was used for the anodizing tank. An electrolyte consisting of 10% by volume sulfuric acid and balance water was placed in the tank. A hollow cylinder of graphite serving as a cathode lined the inside walls of the beaker. The tin oxide resistor which served as the anode was mounted in the beaker with the axis of the cylindrical resistor concentric to the axis of the cylindrical graphite cathode. Both the graphite cathode and the resistor anode were completely immersed in the electrolyte solution. A Crossover Mode power supply was electrically connected to the anodizing apparatus with positive connected to the anode and negative to the cathode. The Crossover Mode was then activated to anodize the tin oxide resistor film utilizing the following parameter values:

Initial current ma 100 Final current ma 60 Initial voltage v 3 Crossover voltage v- 9 Run time min 60 As will be recalled, the thickness of the initial tin oxide film was 6000 A. and the resistance of the resistor was 38.91 ohms. After the above anodization procedure, the thickness of the resistor film was 3400 A. and th thickness of the insulating anodic coating surrounding the conductive resistor film was 2600 A. The resistance of the resistor film after anodization was 80.40 ohms. The resistance of the anodic coating was infinite. It is believed that the reason for the unexpected infinite resistance of the anodic coating is due to the outer portion of the original conducting film being converted to an essentially stoichiometric form.

It is therefore apparent that by varying the parameters utilizedduring the anodization and by making suitable measurements of the resistor characteristics, a metal oxide resistor may be adjusted to a desired resistance value and at the same time have a desirable coating formed thereon. This method of simultaneously adjusting the resistance value of the resistor and forming a nonreactive protective coating overcomes each of the disadvantages of prior methods as set forth hereinabove.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the impending claims the invention may be practiced otherwise than as specifically described.

I claim:

1. A film resistor comprising, an inert nonconducting substrate; a conducting tin oxide film on said substrate; and a nonconducting anodized tin oxide coating on said conducting tin oxide film.

2. A method of making a tin oxide film resistor of precise resistance value comprising, the steps of depositing a conducting tin oxide film on an inert, nonconductive substrate; and electrolytically anodizing said conducting tin oxide film to form a nonconducting anodized layer on said film.

References Cited UNITED STATES PATENTS 3,108,019 10/ 1963 Davis 117-62 XR 3,159,556 12/1964 McLean et a1 204-56 XR 3,180,807 4/1965 Quinn 20456 XR 3,223,601 12/1965 George 20456 3,258,413 6/1966 Pendergast 338-308 XR 3,261,082 7/1966 Maissel et a1. 338-308 XR 3,285,836 11/1966 Maissel et al 204-38 XR WILLIAM L. JARVIS, Primary Examiner I US. Cl. X.R. 117-62, 211, 215; 338-308 

